It took about 10 years to complete the process and was the ultimate act of self-denial: enlightenment through physical punishment. There were three phases. Each phase lasted 1000 days, and involved stricter and stricter eating habits and physical hardship. The goal being to reduce the fat content and slow or prevent the flesh from decaying after death.
1000 days of nuts and seeds. 1000 days of bark and roots. Then 1000 days of tea from a sap used to lacquer furniture. Then the monk was buried alive with a breathing tube and a bell. The monk rang the bell once per day to show he was still alive. When the ringing stopped the tube was removed.
You can actually go to northern Japan and see these monks on display. Here are some pictures.
Oh the things you learn when you google "bizarre traditions." Not that someone else's cultural heritage is bizarre, but...